1. Field of the Invention
The present invention generally relates to a method for producing a permanent magnet that is applicable for use in motors and actuators of various types, and more particularly, the present invention relates to an iron-based rare earth magnet including multiple ferromagnetic phases and a method for producing such a novel magnet.
2. Description of the Related Art
Recently, it has become more and more necessary to further improve the performance of, and further reduce the size and weight of, consumer electronic appliances, office automation appliances and various other types of electric equipment. For these purposes, a permanent magnet for use in each of these appliances is required to maximize its performance to weight ratio when operated as a magnetic circuit. For example, a permanent magnet with a remanence Br of 0.5 T or more is now in high demand. Hard ferrite magnets have been used widely because magnets of this type are relatively inexpensive. However, the hard ferrite magnets cannot achieve the high remanence Br of 0.5 T or more.
An Smxe2x80x94Co type magnet, produced by a powder metallurgical process, is currently known as a typical permanent magnet that achieves the high remanence Br of 0.5 T or more. However, the Smxe2x80x94Co type magnet is expensive, because Sm and Co are both expensive materials.
As for the Ndxe2x80x94Fexe2x80x94B type magnet on the other hand, the magnet is mainly composed of relatively inexpensive Fe (typically at about 60 wt % to 70 wt % of the total weight), and is much less expensive than the Smxe2x80x94Co type magnet. Examples of other high-remanence magnets include an Ndxe2x80x94Fexe2x80x94B type sintered magnet produced by a powder metallurgical process and an Ndxe2x80x94Fexe2x80x94B type rapidly solidified magnet produced by a melt quenching process. An Ndxe2x80x94Fexe2x80x94B type sintered magnet is disclosed in Japanese Laid-Open Publication No. 59-46008, for example, and an Ndxe2x80x94Fexe2x80x94B type rapidly solidified magnet is disclosed in Japanese Laid-Open Publication No. 60-9852, for instance. Nevertheless, it is still expensive to produce the Ndxe2x80x94Fexe2x80x94B type magnet. This is partly because huge equipment and a great number of manufacturing and processing steps are required to separate and purify, or to obtain by reduction reaction, Nd, which usually accounts for 10 at % to 15 at % of the magnet. Also, a powder metallurgical process normally requires a relatively large number of manufacturing and processing steps by its nature.
Compared to an Ndxe2x80x94Fexe2x80x94B type sintered magnet formed by a powder metallurgical process, an Ndxe2x80x94Fexe2x80x94B type rapidly solidified magnet can be produced at a lower cost by a melt quenching process. This is because an Ndxe2x80x94Fexe2x80x94B type rapidly solidified magnet can be produced through relatively simple process steps of melting, melt quenching and heat treating. However, to obtain a permanent magnet in bulk by a melt quenching process, a bonded magnet should be formed by compounding a magnet powder, made from a rapidly solidified alloy, with a resin binder. Accordingly, the magnet powder normally accounts for at most about 80 volume % of the molded bonded magnet. Also, a rapidly solidified alloy, formed by a melt quenching process, is magnetically isotropic.
For these reasons, an Ndxe2x80x94Fexe2x80x94B type rapidly solidified magnet produced by a melt quenching process has a remanence Br lower than that of a magnetically anisotropic Ndxe2x80x94Fexe2x80x94B type sintered magnet produced by a powder metallurgical process.
As disclosed in Japanese Laid-Open Publication No. 1-7502, a technique of adding, in combination, at least one element selected from the group consisting of Zr, Nb, Mo, Hf, Ta and W and at least one more element selected from the group consisting of Ti, V and Cr to the material alloy effectively improves the magnetic properties of an Ndxe2x80x94Fexe2x80x94B type rapidly solidified magnet. When these elements are added to the material alloy, the magnet has increased coercivity HcJ and anticorrosiveness. However, the only known effective method of improving the remanence Br is increasing the density of the bonded magnet. Also, where an Ndxe2x80x94Fexe2x80x94B type rapidly solidified magnet includes a rare earth alloy at 6 at % or more, a melt spinning process, in which a melt of its material alloy is ejected against a chill roller, has often been used in the prior art to rapidly cool and solidify the material alloy at an increased rate.
As for an Ndxe2x80x94Fexe2x80x94B type rapidly solidified magnet, an alternative magnet material was proposed by R. Coehoom et al., in J. de Phys, C8, 1998, pp. 669-670. The Coehoom material has a composition including a rare earth element at a relatively low mole fraction (i.e., around Nd3.8Fe77.2B19, where the subscripts are indicated in atomic percentages); and an Fe3B phase as its main phase. This permanent magnet material is obtained by heating and crystallizing an amorphous alloy that has been prepared by a melt quenching process. Also, the crystallized material has a metastable structure in which soft magnetic Fe3B and hard magnetic Nd2Fe14B phases coexist and in which crystal grains of very small sizes (i.e., on the order of several nanometers) are distributed finely and uniformly as a composite of these two crystalline phases. For that reason, a magnet made from such a material is called a xe2x80x9cnanocomposite magnetxe2x80x9d. It was reported that such a nanocomposite magnet has a remanence Br as high as 1 T or more. But the coercivity HcJ thereof is relatively low, i.e., in the range from 160 kA/m to 240 kA/m. Accordingly, this permanent magnet material is applicable only when the operating point of the magnet is 1 or more.
It has been proposed that various metal elements be added to the material alloy of a nanocomposite magnet to improve the magnetic properties thereof. See, for example, Japanese Laid-Open Publication No. 3-261104, Japanese Patent Publication No. 2727505, Japanese Patent Publication No. 2727506, PCT International Publication No. WO 003/03403 and W. C. Chan et. al., xe2x80x9cThe Effects of Refractory Metals on the Magnetic Properties of xcex1-Fe/R2Fe14B-type Nanocompositesxe2x80x9d, IEEE Trans. Magn. No.5, INTERMAG. 99, Kyongiu, Korea, pp. 3265-3267, 1999. However, none of these proposed techniques are reliable enough to always obtain a sufficient xe2x80x9ccharacteristic value per costxe2x80x9d. More specifically, none of the nanocomposite magnets produced by these techniques realizes a coercivity high enough to actually use it in various applications. Thus, none of these magnets can exhibit commercially viable magnetic properties.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method for producing an iron-based alloy permanent magnet with excellent magnetic properties at a low cost, and provide a permanent magnet that achieves a coercivity HcJ that is high enough to actually use the magnet in various applications (e.g., HcJxe2x89xa7600 kA/m) while maintaining a remanence Br of about 0.8 T or more.
According to a preferred embodiment of the present invention, a method of making a material alloy for an iron-based rare earth magnet includes the step of preparing a melt of an iron-based rare earth material alloy having a composition represented by the general formula (Fe1-mTm)100-x-y-z-n(B1-pCp)xRyTizMn. In this formula, T is at least one element selected from the group consisting of Co and Ni; R is at least one element selected from the group consisting of Y (yttrium) and the rare earth elements; and M is at least one element selected from the group consisting of Al, Si, V, Cr, Mn, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Ag, Hf, Ta, W, Pt, Au and P. The mole fractions x, y, z, m, n and p preferably satisfy the inequalities of: 10 at % less than xxe2x89xa625 at %; 6 at %xe2x89xa6y less than 10 at %; 0.5 at %xe2x89xa6zxe2x89xa612 at %; 0xe2x89xa6mxe2x89xa60.5; 0 at %xe2x89xa6nxe2x89xa610 at %; and 0xe2x89xa6pxe2x89xa60.25, respectively. The method further includes the step of feeding the melt of the material alloy onto a guide, which includes a guide surface that defines an angle of about 1 to about 80 degrees with respect to a horizontal plane, thereby moving the melt onto a region where the melt comes into contact with a chill roller. The method further includes the step of rapidly cooling the melt using the chill roller to make a rapidly solidified alloy including an R2Fe14B phase.
In one preferred embodiment of the present invention, the cooling step may include the step of adjusting a flow width of the melt to a predetermined size in an axial direction of the chill roller using the guide.
In another preferred embodiment of the present invention, the rapidly solidified alloy may be made within a reduced-pressure atmosphere.
Then, the atmospheric gas preferably has its pressure controlled at between about 0.13 kPa and about 100 kPa.
In still another preferred embodiment, the rapidly solidified alloy produced in the cooling step may include the R2Fe14B phase at about 60 volume percent or more.
In yet another preferred embodiment, in the cooling step, a surface velocity of the chill roller may be controlled at between about 5 m/sec and about 26 m/sec, and a feeding rate per unit width of the melt may be controlled at about 3 kg/min/cm or less.
In yet another preferred embodiment, the method may include the step of forming a structure in which three or more crystalline phases, including at least the R2Fe14B phase and xcex1-Fe and ferromagnetic iron-based boride phases, exist; an average crystal grain size of the R2Fe14B phase is between about 20 nm and about 200 nm; and an average crystal grain size of the xcex1-Fe and boride phases is between about 1 nm and about 50 nm.
In yet another preferred embodiment, an iron-based boride phase with ferromagnetic properties may exist around a grain boundary or sub-boundary of the R2Fe14B phase.
Preferably, the structure is formed by subjecting the rapidly solidified alloy to a heat treatment for crystallization. After the heat treatment, the R2Fe14B phase constitutes about 65 vol % to about 85 vol % of the alloy.
In that case, the heat treatment preferably includes a step of keeping the rapidly solidified alloy heated to a temperature between about 550xc2x0 C. and about 850xc2x0 C. for approximately 30 seconds or more.
More preferably, the method further includes the step of pulverizing the rapidly solidified alloy before subjecting the rapidly solidified alloy to the heat treatment.
In yet another preferred embodiment, the iron-based boride phase may include Fe3B and/or Fe23B6.
In yet another preferred embodiment, the element M always includes Nb. In this particular preferred embodiment, the melt of the material alloy including Nb has a liquidus temperature lower by about 10xc2x0 C. or more than that of another iron-based rare earth magnet material alloy that has substantially the same composition as the material alloy including Nb but that includes substantially no Nb.
More specifically, the material alloy preferably includes Nb in an amount between about 0.1 at % and about 3 at %.
In yet another preferred embodiment, an atomic ratio p of C in the general formula preferably satisfies the inequality of 0.01xe2x89xa6p less than 0.25.
In yet another preferred embodiment, before the melt is fed onto the guide, the melt preferably has its kinematic viscosity controlled at approximately 5xc3x9710xe2x88x926 m2/sec or less.
Where 0.01xe2x89xa6p less than 0.25, a compound phase, which precipitates first while the melt is being rapidly cooled and solidified, preferably has its solidification temperature decreased by about 5xc2x0 C. or more compared to a melt of another material alloy with an atomic ratio p of about 0.
In that case, the compound phase that precipitates first while the melt is being rapidly cooled and solidified in the cooling step may be a titanium boride compound.
In yet another preferred embodiment, the cooling step may be performed by rotating the chill roller, which preferably has a centerline roughness Ra of about 20 xcexcm or less on its surface, at a surface velocity of approximately 10 m/sec or more.
In yet another preferred embodiment, a melt flow quenching rate, at which each flow of the melt is rapidly cooled and solidified by the chill roller in the cooling step, may be controlled at about 0.7 kg/min or more but less than about 4 kg/min.
In yet another preferred embodiment, each flow of the melt may have its width controlled in the cooling step by the guide at about 5 mm or more but less than about 20 mm.
In yet another preferred embodiment, the melt may have its kinematic viscosity controlled at approximately 5xc3x9710xe2x88x926 m2/sec or less.
In yet another preferred embodiment, the guide may have its surface temperature kept at approximately 300xc2x0 C. or more so that the melt has a kinematic viscosity of no greater than about 5xc3x9710xe2x88x926 m2/sec.
In yet another preferred embodiment, the rapidly solidified alloy may have a thickness of between about 50 xcexcm and about 200 xcexcm.
In yet another preferred embodiment, the guide may be made of a material that includes Al2O3 at about 80 volume percent or more.
In yet another preferred embodiment, the chill roller may include a base made of a material with a thermal conductivity of approximately 50 W/m/K or more.
In that case, the base of the chill roller is preferably made of carbon steel, tungsten, iron, copper, molybdenum, beryllium or a copper alloy, or other suitable material.
Optionally, the base of the chill roller may have its surface plated with chromium, nickel or a mixture thereof, or other suitable material.
Another preferred embodiment of the present invention provides a method for producing an iron-based permanent magnet that includes the steps of preparing the material alloy for the iron-based rare earth magnet by the inventive method of making a material alloy according to the preferred embodiments of the present invention described above, and subjecting the material alloy for the iron-based rare earth magnet to a heat treatment.
In another preferred embodiment of the present invention, an inventive method for producing a bonded magnet includes the steps of preparing a powder of the material alloy for the iron-based rare earth magnet by the inventive method of making a material alloy or a powder of the iron-based permanent magnet by the inventive method for producing an iron-based permanent magnet according to the preferred embodiments described above, and processing the powder into the bonded magnet.
A rapidly solidified alloy according to various preferred embodiments of the present invention preferably has a composition represented by the general formula (Fe1-mTm)100-x-y-z-nQxRyTizMn. In this formula, T is at least one element selected from the group consisting of Co and Ni; Q is at least one element selected from the group consisting of B and C; R is a rare earth element; and M is at least one element selected from the group consisting of Al, Si, V, Cr, Mn, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Hf, Ta, W, Pt, Pb, Au and Ag. The mole fractions x, y, z, m and n preferably satisfy the inequalities of: 10 at % less than xxe2x89xa620 at %; 6 at %xe2x89xa6y less than 10 at %; 0.5 at %xe2x89xa6zxe2x89xa66 at %; 0xe2x89xa6mxe2x89xa60.5; and 0 at %xe2x89xa6nxe2x89xa65 at %, respectively. The alloy preferably has a thickness of between about 50 xcexcm and about 200 xcexcm. In this alloy, a crystal structure has been formed on each of two surfaces thereof that cross a thickness direction at right angles.
In one preferred embodiment of the present invention, the crystal structure may include a ferromagnetic boride phase with an average crystal grain size of between about 1 nm and about 50 nm, and an R2Fe14B phase with an average crystal grain size of between about 20 nm and about 200 nm.
In another preferred embodiment of the present invention, an amorphous portion is interposed between the crystal structures on the two surfaces.
In this particular preferred embodiment, the alloy preferably has a thickness of about 80 xcexcm or more.
Another rapidly solidified alloy according to a preferred embodiment of the present invention has a composition represented by the general formula
(Fe1-mTm)100-x-y-z-nQxRyTizMn. In this formula, T is at least one element selected from the group consisting of Co and Ni; Q is at least one element selected from the group consisting of B and C; R is a rare earth element; and M is at least one element selected from the group consisting of Al, Si, V, Cr, Mn, Cu, Zn, Ga, Zr, Nb, Mo, Hf, Ta, W, Pt, Pb, Au and Ag. The mole fractions x, y, z, m and n preferably satisfy the inequalities of: 10 at % less than xxe2x89xa620 at %; 6 at %xe2x89xa6y less than 10 at %; 0.5 at %xe2x89xa6zxe2x89xa66 at %; 0xe2x89xa6mxe2x89xa60.5; and 0 at %xe2x89xa6nxe2x89xa65 at %, respectively. The alloy preferably has a thickness of between about 60 xcexcm and about 150 xcexcm and a recoil permeability of between about 1.1 and about 2.
A magnet powder according to a preferred embodiment of the present invention has a composition represented by the general formula (Fe1-mTm)100-x-y-z-nQxRyTizMn. In this formula, T is at least one element selected from the group consisting of Co and Ni; Q is at least one element selected from the group consisting of B and C; R is a rare earth element; and M is at least one element selected from the group consisting of Al, Si, V, Cr, Mn, Cu, Zn, Ga, Zr, Nb, Mo, Hf, Ta, W, Pt, Pb, Au and Ag. The mole fractions x, y, z, m and n preferably satisfy the inequalities of: 10 at % less than xxe2x89xa620 at %; 6 at %xe2x89xa6y less than 10 at %; 0.5 at %, zxe2x89xa66 at %; 0xe2x89xa6mxe2x89xa60.5; and 0 at %xe2x89xa6nxe2x89xa65 at %, respectively. The powder preferably has a mean particle size of between about 60 xcexcm and about 110 xcexcm. A ratio of a major-axis size of the powder to a minor-axis size thereof is between about 0.3 and about 1. The powder preferably has a coercivity HcJ of approximately 600 kA/m or more.
Other features, elements, processes, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.